Related arts are disclosed by U.S. Pat. No. 4,533,937 and JP Patented No. 2,556,810. First, the prior art disclosed by U.S. Pat. No. 4,533,937 is described hereinafter. FIG. 12 is a block diagram illustrating a first example of a video mixing apparatus carrying out a conventional chromakey process.
In this first example, a component of a foreground object is taken out from a source video signal, and mixed with background video signal. The source video signal is generated by shooting the foreground object in front of a monochromatic screen. This is known as a chromakey process.
As shown in FIG. 12, this video mixing apparatus performs as follows: Key signal generator 1101 generates key signal K using source video signal Vs and a screen reference color. Screen reference color memory 1102 stores screen reference color Vr designated by a user. Mixing processor 1103 mixes source video signal Vs with background video signal Vz based on key signal K and screen reference color Vr, thereby outputting mixed video signal Vm.
Reference marks Vs, Vz, Vr and Vm are three-dimensional color vectors of which respective luminance component Y, blue color difference component Cb, and red color difference component Cr are (Sy, Scb, Scr), (Zy, Zcb, Zcr), (Ry, Rcb, Rcr), (My, Mcb, Mcr), and reference mark K is a scalar value taking 0≦K≦1.
Key signal generator 1101 generates key-signal K, for instance, as follows: Formula (1) indicates distance “d” (d≧0) on Cb−Cr plane between source video signal Vs and screen reference color Vr. Output signal K is a function of input signal “d” and is saturated by base-clip-level “db” and peak-clip-level “dp”. This is shown in formula (2).d=√{square root over (Scb−Rcb)2+(Scr−Rcr)2)}{square root over (Scb−Rcb)2+(Scr−Rcr)2)}  (1)K=Clip(d,db,dp)  (2)where function “Clip” in formula (2) produces an output saturated with respect to the input signal at base clip level “b” and peak clip level “p” as shown in FIG. 13. Function “Clip” carries out the following calculations:i) When d≦b, Clip(d,b,p)=0  (3a)ii) When b<d<p, Clip(d,b,p)=((d−b)/(p−b)  (3b)iii) When p≦d, Clip(d,b,p)=1  (3c)
An example case, where base-clip-level “db” and peak-clip-level “dp” take constant values, is described hereinafter. Key signal generator 1101, as shown in FIG. 14, generates key signal K based on a distribution where two circles having respective radiuses “db” and “dp” draw boundaries. In other words, regarding distance “d”,the following three cases are available:                i) When “d” is inside 140 of the circle having radius “db”, K=0.        ii) When “d” is outside 142 of the circle having radius “dp”, K=1.        iii) When “d” is at a place other than the above two cases, 0<K=(d−db)/(dp−db)<1.        
As such, in this first example, key signal generator 1101 determines key signal K using only a color difference value of the source video signal.
FIG. 15 shows an example of mixing processor 1103, which mixes source video signal Vs with background video signal Vz based on key signal K, thereby outputting mixed video signal Vm. Adder 1401 finds a component of foreground object by subtracting (1−K)Vr from source video signal Vs, where (1−K)Vr is a screen component calculated by multiplier 1402. Another adder 1404 outputs mixed video signal Vm by adding (1−K)Vz to foreground object component Vf, where (1−K)Vz is a background video component calculated by another multiplier 1403.
In the structure shown in FIG. 15, mixing processor 1103 outputs mixed video signal Vm according to the following formulas (4):Y component: My=Sy−(1−K)Ry+(1−K)Zy  (4a)Cb component: Mcb=Scb−(1−K)Rcb+(1−K)Zcb  (4b)Cr component: Mcr=Scr−(1−K)Rcr+(1−K)Zcr  (4c)
Next, the prior art disclosed by JP Patented No. 2,556,810 is described hereinafter. FIG. 16 is a block diagram illustrating a second example of the conventional video mixing apparatus. This apparatus also carries out the chromakey process. This second example differs from the first one in the following point: Key signal generator 1501 outputs two key signals Kc (color-canceling key signal) and Km (mixing key signal). Mixing processor 1503 calculates the mixing based on the two key signals Kc, Km. In this example, the mixing is carried out according to the following formulas:Y component: My=Sy−(1−Km)Ry+(1−Km)Zy  (5a)Cb component: Mcb=Scb−(1−Kc)Rcb+(1−Km)Zcb  (5b)Cr component: Mcr=Scr−(1−Kc)Rcr+(1−Km)Zcr  (5c)
As formula (5) tells, in this example, color-canceling key signal Kc used for subtracting a color difference component can have a value different from mixing process key signal Km. Thus, it can be set that adding of a background video signal component is carried out in a narrow region on a color space and weakening of a screen color is carried out in a rather greater region. Therefore, at a portion where the screen color mixes into the foreground object, the screen color can be weakened without any transparency in the foreground object.
However, the video mixing apparatus of this example generates a key signal using only color difference information of the source video signal, thus, it sometimes cannot separate properly the foreground object component from the screen component.
Hereinafter described is a way of generating a key signal from the source video shown in FIG. 17. Respective pixels constituting the source video shown in FIG. 17 are projected to a plane vertical to a color difference plane. FIG. 18 shows this projection. The pixels constituting the foreground object are distributed in the vicinity of region F 1701 in FIG. 18, while the pixels constituting the screen are distributed in the vicinity of region X 1702. Between these two regions, the pixels constituting the edge of foreground object are distributed, and the pixels are mixed with both the components of foreground object and the screen.
In this example, it is difficult to set a proper edge (boundary) for generating a key signal. For instance, when a boundary face—separating a region of K=1 and another region of K<1—is set as a boundary face “a” denoted with 1801 in FIG. 19, the component of foreground object is weakened. On the other hand, when the boundary face is set as boundary face “b” denoted with 1802 in FIG. 19, the screen color invades the edge in the mixed video.
In the way discussed above, when the mixing process is carried out by the calculations shown in formulas (4), noises (uneven lighting, scratches, stains, and the like) in the screen appear in the mixed video, thereby lowering the picture quality. This is because vector Vr having a constant value is used as a subtracting vector for removing the screen component.
Basically, in a pixel at which the key signal generator outputs key signal K=0, mixed video signal Vm− background video signal Vz should be satisfied. In other words, regarding a luminance component, the term of Sy−(1−K)Ry in (4a) should take “0” (zero) and My=Zy should be satisfied. However, since the screen has some noises, Sy≠Ry, thus error of Sy−Ry appears in the mixed video.